This invention relates to expansible shafts.
Such shafts have an axially extending housing and a core engager which moves radially from a retracted position within the housing to an expanded position in which the core engager extends beyond the outer housing wall. For example, Peterson U.S. Pat. No. 4,254,920 (assigned to the assignee of this application) discloses a fluid-actuated shaft having an operating rod that is mounted co-axially with and moves axially relative to the housing. Springs move actuators to force the core engagers radially outwardly into engagement with a surrounding core and a fluid-activated piston drives the operating rod in the other direction to retract them. There are also shafts which use mechanical means (e.g., Acme screws) to expand actuators.
It is desirable that there be a positive, essentially fail-safe mechanism to retract the core-engagers. Springs and similar mechanisms can jam internally of the shaft, and also may not provide sufficient force if the core engagers have become embedded in the surrounding core.